Happy Mole Day!

Mole Day is celebrated every year on 10/23, between 6:02am and 6:02pm. And why is this day special? Because a mole is approximately 6.02×1023. OK, sure, but what’s it mean?

The standard definition of “mole” is the number of atoms of carbon-12 in 12g. (carbon-12 has 6 protons and 6 neutrons, giving it an atomic weight of 12 – more or less) So, you take 12 g on pure carbon-12, count the atoms and you get a mole. This is also known as Avogadro’s Number.

Big deal, right? So you can figure out the number of carbon atoms in 12g. What good is that? Well, it also works out to the number of atoms of X grams of any substance where that substance has an atomic weight of X. That is, it has a number of protons and neutrons adding up to X so, if you weigh out X grams of it, you’d have a mole of atoms of it. For example…

A mole of… weighs…
oxygen-16 16g
chromium-52 52g
uranium-235 235g


You can extend this to molecules too, since molecules are just combinations of atoms stuck together. You might notice that “molecule” is a derivation of “mole”. It means “little mole” (sort of).

Let’s take C8H10N4O2, for example. Like the formula says, it’s got 8 carbon atoms, 10 hydrogen, 4 nitrogen and 2 oxygen. So the total molecular weight is…

C 12 x 8 = 96
H 1 x 10 = 10
N 14 x 4 = 56
O 16 x 2 = 32
Total 194

So 6.02×1023 molecules of caffeine weighs 194g. OK, it’s actually 194.19g because of a few naturally-occurring isotopes (atoms with fewer or more neutrons than usual), which throws it off a bit.

Now all this is very handy for chemists because you can work out how much of different substances you need for a chemical reaction to work properly.

For example, if you want to combine acetic acid (molecular weight 60) with sodium bicarbonate (molecular weight 84), you’d need to mix 60g of acetic acid with 84g of sodium bicarbonate to get 82g of sodium acetate, 18g of water and 44g of carbon dioxide. This is the classic vinegar and baking soda reaction that has been powering papier mache volcanoes for decades.

Note that the total mass going in and coming out is 144g. No mass lost or gained. It’s all the same atoms, just rearranged into different configurations. Neat, huh?

Also note that, while baking soda is pure sodium bicarbonate, vinegar is watered down acetic acid. It’s usually a 4% solution, so you’d need 25 times the weight of vinegar (1500g) to make the reaction work properly, and you’ll end up with a lot more leftover water. Or, you know, you could use less baking soda. Either way works, as long as the ratios stay the same.

And that’s pretty much what chemists do. It doesn’t matter if you’re mixing 60g of acetic acid with 84g of sodium bicarbonate in a beaker or 60kg with 84kg in a factory. You’re going to get the same reaction.

So, that’s why moles are so important to chemists, which is why chemistry geeks celebrate Mole Day. And also, Santavogadro visits all the good boy and girl chemists of the world and stuffs their Erlenmeyer flasks with 40% (by molecular weight) concentrations of ethanol. Hey, it could happen.

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Steve D

Steve D

Steve consists of approximately 60% water and 40% organic molecules, arranged in a configuration that is, among over things, capable of describing itself in this manner.

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